Several types of organic light emitting electrochemical cells (OLECs) that have homogenous active layer compositions are known in the art. OLECs provide stable, low cost organic light emitting devices requiring low operating voltage. In their typical form, OLECs, such as those described in U.S. Pat. No. 5,682,043, are deposited from a single solution containing light emitting and charge transporting conjugated polymers, for example polyphenylene vinylene polymers, polythiophene-based polymers or polyfluorene-based polymers, in combination with an electrolyte system which typically includes mobile ionic dopants, for example lithium triflate along with electrolyte forming materials such as polyethylene oxide or similar. Low voltage charge injection in these devices, and relatively high quantum efficiency under bias, are generally attributed to the electric field driven redistribution of cation and ions from the electrolyte at the cathode and anode interfaces of the OLEC with its adjacent electrode. Ionic dopant redistribution serves to electrochemically dope the conjugated semiconductor active layer materials near the interface, leading to strong band bending and narrowing of the stateless injection barrier between electrons in filled states in the conductive electrode and corresponding unfilled molecular energy levels in the active layer semiconductors. This effectively lowers the bias required to inject electrons and holes into the active layer, leading to lower voltage electroluminescence when significant and relatively balanced electron and hole currents recombine at luminescent sites in the active layer.
In contrast to conventional organic light emitting diodes (such as devices shown in U.S. Pat. No. 4,539,507), which rely on near ohmic matching of the work function of the electrodes and the energy levels within the active layers of the device, and are often composed of multiple layers of organic semiconductor-based materials with different energy levels, the conventional OLEC has the advantage that charge injection at low bias can be achieved from metals of less well-matched work function, than is required by a conventional OLED. Essentially all previous LEC devices have been based on a single light-emitting polymer, even where some structuring of doping has been considered. Examples can be found in U.S. Pat. No. 6,326,091.
It is particularly advantageous to eliminate the need for low work function cathode materials in OLECs at the electron-injecting interface. Low work function cathode metals such as Ca, Li, Cs, Ba, and Mg, which are commonly used in conventional approaches, including polymer OLEDs (such as devices shown in U.S. Pat. No. 5,247,190), are typically unstable to oxidation or other degradation mechanisms, particularly in the presence of moisture or oxygen. This constrains the processing environments for devices containing these materials. These low work function metals also have higher encapsulation requirements and/or shorter storage lifetimes as water or oxygen leakage into the device leads to spontaneous cathode degradation. Further references can be found in U.S. Pat. No. 6,522,067.
Recently, significant attention has been given to printing- and coating-based approaches for the fabrication of semiconductor thin film and, in particular, organic semiconductor thin films devices. These printing and coating-based manufacturing approaches permit lower cost, higher volume production of device over larger areas and on flexible substrates. The doping concept is particularly attractive for manufacturing light emitting organic devices as it permits the use of printable, air stable, and air processable cathode materials (for example U.S. Pat. No. 6,605,483). However, high levels of doping can reduce luminance efficiency due to doping-induced quenching of luminescence. Also, side reactions of dopants, which are exacerbated at high concentrations, can lead to accelerated degradation of device performance. Generally speaking, it may be more difficult to inject one charge type than the other in a given active layer medium. Often, as is the case for the combination of alkoxy substituted PPVs such as ‘Super Yellow’ (SY) as sold by Merck OLED GMBH, it is electron injection that is more difficult to achieve than hole injection, particularly when higher work function and more air stable cathodes are used. This has also been observed in polyfluorene-based LECs.
In addition to low cost processing of single active layer devices (i.e. excluding conducting polymer hole injection layers) with homogenous, as deposited compositions, such as those described in U.S. Pat. No. 5,682,043 and U.S. Pat. No. 6,605,483, printing also allows for the easy deposition of multilayered active layer structures. Light-emitting electrochemical cells (LECs) can be printed with good optoelectronic performance at thicknesses ranging from ˜150 nm to over 1 micron. This total thickness can be built up by a number of successive print passes. The present inventors have recognised that this printing approach provides an opportunity to vary composition between printed layer passes including variations in light emitters, charge transport, and ionic dopant functionality through the thickness of the device. By using different inks for different printed layers, stratified devices can be produced. In some cases, drying of printing solvent between layer depositions may be effective in preventing redissolution of underlying layers that would otherwise degrade the desired vertical composition profile. In other cases, cross-linking agents, the use of active materials or additive materials with orthogonal solubility in solvents used for deposition of successive layers may also be used to limit inter-layer solubility.
In U.S. Pat. No. 6,326,091, Schoo et al., propose a type of multilayer structure built solely around a mobile and immobile ion concept in which there are only mobile or immobile ions for one of the charge types. We have found, as in U.S. patent application Ser. No. 12/557,316 to Chen, et al. that mixtures of ion mobilities and properties are advantageous. Furthermore, U.S. Pat. No. 6,326,091 specifies ionic mobility and compositions outside the ranges applicable in the present invention. For example, U.S. Pat. No. 6,326,091 specifies ionic mobilities for the immobile ion as being higher than 1e-19 cm2/Vs. For devices of interest to this invention, ranging from 100 nm to 500 nm in thickness and at operating voltages preferably below 24V and more preferably near or below 10V typical operating conditions, the lowest immobile ion mobility would be ˜5e-17 and more likely >1e-15 cm2/vs. This assumes that useful biasing cycles would be <24 hr and more likely <1 hr. In this case, the acceptable minimum mobility required here is actually similar to the ‘mobile’ ion of U.S. Pat. No. 6,326,091. In addition, U.S. Pat. No. 6,326,091 specifies that the second layer of the multilayer contain mobile ions and immobile ions in equal amounts. This is a specification that is not required in the present invention.
Multilayer device structures, first investigated in Merck Super Yellow (SY) based printed light-emitting electrochemical cell (LEC) devices, have been shown to be very effective in increasing lifetimes, efficiencies and in some cases turn-on times in printed devices derived from screen printing and gravure printing. In particular, they have been particularly effective in improving polyfluorene Light-Emitting Polymer (LEP)-based devices where there has been the additional factor of allowing the use of LEP's with varying electronic energy levels and charge transport levels that would not otherwise be printable due to poor print morphology and yield issues. The polyfluorene series of materials also allowed the more advanced multilayer configurations, which include stepwise variation of transport properties in the devices stack, which was not possible in the simple Super Yellow (SY) LEP case. Although fabricating multilayers introduces complexity due to the need for a second LEP ink, this is not a significant burden in manufacturing as devices are typically printed in several layers with both screen or gravure printing, even if they contain a single LEP formulation throughout the device.
Simple doping multilayers where neutral, compensated within each layer as deposited, dopant concentrations are peaked near the cathode interface can significantly improve device performance. However, further improved results occur when doping gradients are also combined with electronic transport gradients through the thickness of the devices. Multilayer printed LEC devices based on Sumation light-emitting polymers with air printed cathodes exceeding 14,000 hours lifetime (100 Cd/m2) have been demonstrated in this way and represent the longest lifetime printed LEC devices known to the inventors. Advantageous structuring of ionic dopant, ionic support materials and semiconducting polymer content can also result in improved turn-on kinetics of OLECs. Note that ion-motion related slow turn-on effects are a limitation in some conventional OLECs.
Overall, the multilayer work with SY and polyfluorene-based LEP materials supports the general conclusion that devices are cathode injection limited to some degree. In all cases, heavier doping of the near cathode region, even with a common LEP, results in improved devices. All recent multilayer device work incorporates higher doping levels in the near-cathode layers unless otherwise specified. The objective of the present disclosure is to teach device active layer structures with special properties at the regions adjacent to an electrode. In the following text, specific reference nomenclature has been used sporadically to refer to certain device structures/materials. For example, the code ‘SWL’ indicates a specific type of white light emitting polymer. Persons skilled in the art will easily understand that this specific nomenclature is for illustrative purposes only, and the scope of the present invention is not limited to the specific device structures/materials denoted by the nomenclature.
Examples of early work in polyfluorene-based multilayers which showed an almost 3× improvement in lifetime and approximately a 50% improvement in external quantum efficiency (EQE) is shown in FIGS. 1A-1D. LEP006 was an early polyfluorene-based LEP derivative with interesting performance levels compared to other candidates available at the time. However, the film morphology was poor. LEP011 used a different backbone monomer, which resulted in significantly improved print quality but poorer voltage performance. The hole transport monomer fractions were similar in these two LEPs. It is possible that the improvement in device performance in this case was related to the improved cathode layer LEP morphology and/or the differences in electronic properties also caused by the LEP011 monomer change that gave advantageous charger transport and/or blocking effects at the LEP006/LEP011 interface.
Lifetime test studies compared blended LEP006+LEP011 all printed device in N2 test environment (FIG. 1A) with devices with getter and in air (FIG. 1B). Lifetimes ranged from <200 h for non-gettered devices to ˜635 hrs for gettered devices. This performance was similar to the single LEP (LEP006 only) level of performance. However, multilayer structures of LEP011 and LEP006 had lifetimes of ˜1000 hrs and 2000 hrs for N2 tested, ungettered devices and air-tested gettered devices, respectively. In FIGS. 1A-1D, the LEPs are described as vendor-specific codes SWL006 and SWL011. Curves A and B in FIG. 1A show voltage-vs-time curves, while curves C and D show brightness-vs-time curves at a current density of 3 mA/cm2. FIG. 1B shows voltage-vs-time curves E, G and I at current densities 2 mA/cm2, 3 mA/cm2 and 4 mA/cm2 respectively. FIG. 1B also shows brightness-vs-time curves F, H and J at current densities 2 mA/cm2, 3 mA/cm2 and 4 mA/cm2 respectively. Multilayer structures of LEP011 and LEP006 had lifetimes of ˜1000 hrs and 2000 hrs for N2 tested, ungettered devices and air-tested gettered devices, respectively. FIG. 1C shows voltage-vs-time curves K and M at current densities 3 mA/cm2, and 2 mA/cm2 respectively for a specific device (without getter). FIG. 1C also shows brightness-vs-time curves L and N at current densities 3 mA/cm2, and 2 mA/cm2 respectively. For gettered devices, FIG. 1D shows voltage-vs-time curves P and R at current densities 2 mA/cm2, and 3 mA/cm2 respectively. FIG. 1D also shows brightness-vs-time curves Q and S at current densities 2 mA/cm2, and 3 mA/cm2 respectively.